This invention relates to a process and apparatus for making a non-woven sheet by flash spinning a plexifilamentary strand, spreading the strand to form a web and oscillating the web and charging the web, and, more particularly, it relates to a process and apparatus for detecting loss of oscillation of the web and signaling said loss to indicate the need for corrective action.
A single position apparatus for use in making nonwoven fibrous sheets of organic synthetic polymers is disclosed in Brethauer et al. U.S. Pat. No. 3,860,369. Farago U.S. Pat. No. 4,537,733, discloses a multiposition apparatus of the type disclosed in Brethauer et al. to produce wide non-woven sheets at greater throughputs. However, as throughputs increase, the potential also increases for blow-ups to occur which in turn cause multiposition spinning machine loss due to knock-down of adjacent positions. More particularly, a blow-up is an occurrence in which flash spun fibers hang in the fibers forwarding device and prevents the fibers from being transported in a gas stream to a woven metal lay down belt. When a blow-up occurs, the position is likely to drop a large bundle of accumulated fibers which knock down nearby spinning positions or become entangled in transporting rolls resulting in a total spinning machine shutdown. Also, blow-ups can be caused by the loss of electrostatic charge on the spun fibers. The loss of electrostatic charge allows the fibers to fly freely above the metal lay down belt due to the loss of electrostatic pinning. The free floating fibers then become entangled in nearby spinning positions generating additional blow-ups.
Continuous visual observation by personnel positioned at strategic locations is required to detect a position blow-up. Often a blown position can go undetected for several seconds which can then cause large clumps of fibers being deposited onto the lay down belt or to flare out and knock down nearby positions and, since the detection of a blow-up requires human intervention, mistakes are often made in shutting down incorrect positions. Other methods of detecting a blow-up can be through video camera observation or light beam disruption, each of which are susceptible to dirt or polymer dust buildup making the device inoperable.
Another method could be the use of an electrostatic detector known as a field mill. A field mill is a device in which an electrostatic charge sensing area is located behind a rotating metal blade similar to a fan blade. The rotation of the grounded blade alternately forces charge to build up and collapse on the sensing area. This rotation of the blade produces an AC voltage on the sensing area proportional to the charge in front of the sensing area. Because electrostatic charge on plastic forwarding devices can build up to many times the charge on the spun filaments, the field mill is limited in detection of only the fiber electrostatic charge. This device can have large errors introduced due to electrostatically charged surfaces nearby and must be gas purged to prevent fiber and polymer entanglement on the rotating blade and sensor.